We like (and dislike) people thanks to projection. That is, when we see traits about a person that we value highly within ourselves, we like then; and when we see things in a person we have distaste for within ourselves,we dislike them. Since you see the breakup as you letting go of your ex, your subconscious translated that to you letting go of the same traits in yourself. This is a two-fold mourning, in that it's also reflecting on you letting go of who you were when you were still with your ex.

And on a second level, it's your subconscious facing a fear that you wouldn't be able to handle in the waking world.